Wireless networks have experienced increased development in the past decade. One of the most rapidly developing areas is mobile ad-hoc networks. Physically, a mobile ad-hoc network includes a number of geographically-distributed, potentially mobile nodes sharing a common radio channel. Compared with other types of networks, such as cellular networks or satellite networks the most distinctive feature of mobile ad-hoc networks is the lack of any fixed infrastructure. The network may be formed of mobile nodes only, and a network is created “on the fly” as the nodes transmit with each other. The network does not depend on a particular node and dynamically adjusts as some nodes join or others leave the network.
The standards governing Wireless Local Area Networks (WLANs) networking products are defined by a suite of specifications issued by the IEEE and known as the IEEE 802.11 standard, incorporated herein by reference in their entirety. The standards define the operation of both the radio PHY layer and the MAC layer including a synchronization mechanism. The synchronization mechanism is used to provide a uniform time base.
For example, a timing synchronization function is operative to keep the timers of all the nodes in the same basic service set (BSS) synchronized. Each node maintains its own local timer. In a conventional WLAN infrastructure network, the access point is the timing master and is operative to implement the timing synchronization function. The access point periodically transmits special frames called beacons that contain a copy of its timing synchronization function timer. The beacons are used by the other nodes to synchronize to the access point. A node always accepts the timing information received in a beacon from the access point servicing its BSS. If the timer of a node is different from the timestamp in the received beacon, the receiving node sets its local timer to the received timestamp value.
For ad-hoc networks, the timing synchronization is typically implemented using a distributed algorithm that is performed by the member nodes. Each node transmits beacons in accordance with an algorithm defined in the 802.11 standard. Each node adopts the timing received from any beacon or probe response that has a timing synchronization function value later than its own timer. Nodes expect to receive beacons at a nominal rate. The interval between beacon transmissions is defined by beacon period parameter of the node. A node sending a beacon sets the value of the timestamp to be equal to the value of the timing synchronization function timer of the node at the time that the first bit of the timestamp is transmitted to the PHY plus the transmitting delays of the node through its local PHY from the MAC-PHY interface to its interface with the wireless medium (i.e. antenna, etc.).
Time Division Multiple Access (TDMA) based ad-hoc networks are prone to synchronization issues due to node movement, radio clock drifting, merging of groups of nodes with different timing references. A timing reference is the beginning time of a common frame. There are many other causes for network synchronization errors.
Various problems exist with correcting timing reference errors. For example, a node can broadcast a timing reference packet to correct the timing errors in the network. But it may take sometime or require continuous flooding of the timing reference information for the timing reference node to know about the error conditions.
Current approaches include flooding the network with a Timing Reference Packet (TRP) but bandwidth consumption is a concern. If the network maintains a network clock then the TRP uses a time stamp to represent the correct timing reference. For example, an 802.11 type of network keeps a common network clock. If there are unstable links, not every node could hear the single flooding of the timing reference packet, and flooding may have to be done multiple times, or periodically. The timing reference packet has to be sent periodically due to clock drifting. If a network clock is not maintained (identity of the timing reference but not the reference time is distributed) then the TRP may contain a unique packet ID, so that whichever node has received the TRP can derive the correct identification of the timing reference node. The timing reference node is notified if a synchronization error is detected or the TRP is periodically flooded. However, a timing reference ambiguity can persist for some time.
Another conventional approach is Ad-hoc Extensions to the 802.15.2 MAC Protocol (developed by Winlab, Rutgers university). The group uses 2 keys in a beacon. A key is similar to the group ID. It is used to indicate synchronization status and membership. Either one of the two can be used to identify the group. The two keys shared by all member nodes have different expiration time stamps. They will be regenerated and re-distributed to all members, if expired. If the group is separated longer than the key expiration period, the current keys circulated in each group will be different. When the sub-groups merge, they will go through the normally required re-synchronization process. However, it will be immediately evident to all nodes in each group that there are two timing reference nodes.
This approach can be complicated when the groups are merging at the same moment as the new keys are being generated and re-distributed in ether, or both groups. Another disadvantage of this approach is the bandwidth consumption by the two keys. In an HP-NET, a key of 20 to 30 bits would consume a significant amount of the bandwidth allocated by each node to network control tasks.
Many references relate to the synchronization of devices in hierarchical wireless networks, where one or several synchronized masters operate as timeservers. U.S. Patent Application Serial No. 20040190487, entitled “Method For Synchronizing A Control Channel To A Working Channel”, filed on Mar. 31, 2003, for example, relates to a method for synchronizing a control channel to a working channel. Moreover, U.S. Pat. No. 6,792,247 relates to a method for synchronizing reception in wireless networks, in which a synchronization packet of a special format is transmitted before a data packet is transmitted. U.S. Pat. No. 6,785,253 and U.S. Pat. No. 6,622,022 also disclose methods for synchronizing hierarchical wireless networks wherein at least one node is selected as the central transfer node or as the main network node which is responsible for providing frame synchronization services. Moreover, U.S. Pat. No. 6,546,026 relates to a method for improving the time synchronization in wireless applications (e.g., TDMA).
Methods for synchronizing cellular wireless networks using GPS timing signals can also be found in U.S. Pat. No. 6,542,754 and U.S. Pat. No. 6,538,600. Moreover, U.S. Pat. No. 6,466,608 relates to a synchronization method that requires defining a hierarchical structure of the network, wherein the synchronization process is controlled by assigned master nodes. Moreover, U.S. Pat. No. 5,812,547 relates to a method for transmitting data packets in a wireless network, without dependence on fixed time slot or a central timing mechanism.
U.S. Pat. No. 6,594,273 relates to a method for communicating in an ad-hoc multihopping network, wherein the network includes active and passive terminals, and wherein only the active terminals participate in routing and synchronization. U.S. Pat. No. 6,807,165 and U.S. Pat. No. 5,699,388 present methods using a unique time source and propagating the synchronization in the network from upstream to downstream terminals. Moreover, U.S. Patent Application Serial No. 20040005902, entitled “System and method for correcting the clock drift and maintaining the synchronization of low quality clocks in wireless networks”, filed Jul. 5, 2002, relates to a method that allows synchronization of terminals at any precision, using a reference clock that operates as a network master.